It is known from the prior art that currents induced in the rotor and conducted via the rotor shaft have a damaging effect on the shaft bearings in electric motors (“bearing currents”). As soon as the induced currents overcome the resistance of the bearing lubricant, they discharge via the bearing and damage the ball raceway, causing melt craters, corrosion and profile damage. This initially leads to bearing noises and a premature total failure.
In order to overcome this, solutions have already been proposed in the prior art, in which solutions the rotor shaft is completely electrically insulated. In this case, the rotor shaft has shoulders in the bearing regions which are extrusion coated with plastics material. Moreover, distance plates made of electrically insulating resin bonded fabric are used for preventing direct contact (on the rotor side and the stator side) between the press-fitted shaft in the rotor and the bearings.
We have found that a disadvantage of the plastics extrusion coating is that this tends to creep or sink in particular under the effect of heat, and does not constitute a robust system in the long term. The use is also restricted, since it is not suitable for drive motors, for example, which exert a pulsating load on the extrusion coating on account of the increased rotational speeds and the additional radial force. A further disadvantage is that the extrusion coating is not only electrically insulating, but also thermally insulating. This in turn leads to an undesirable increase in the bearing temperature. In this case, it is mainly the rotor-side bearing which is affected, since said bearing experiences the highest load of both bearings and is usually located at the end of the stator bush. In an arrangement of this kind, heat can be conducted away only by the stator bush, which has already been heated by the coil.
A further option for preventing bearing currents is the use of hybrid ball bearings, in which ceramic balls prevent electrical conductance. However, this is very costly.
Further background information and examples of devices for discharging charges are described for example in EP1460885 B1 and EP523086 B1, in which annular elements having bristles engage on a shaft in order to discharge static charges via the shaft.